The present invention relates to a double backer for the formation of a double face corrugated web and, more particularly, to an improved system for providing a vacuum holddown force to the web moving through a double backer heating section while minimizing the vertical load.
In a conventional double backer, a liner web is brought into contact with the glued flute tips of a single face corrugated web, and the freshly glued double face web is then passed over the coplanar surfaces of a number of serially arranged heating units, usually steam heated, to cause the starch-based glue to cure and to drive moisture from the web. For many years, double face web travel over the flat heated surfaces of the heating units was typically provided by a wide driven holddown belt in direct contact with the upper face of the corrugated web. The top face of the holddown belt, in turn, is held in contact with the moving web by any of several types of load or force applying devices. For example, the holddown belt may be engaged by a series of weighted ballast rollers, or it may be forced into contact with the web by air pressure from a system of nozzles positioned over the belt, or an arrangement of inflatable air bladders may be used to press the moving holddown belt into contact with the web.
The use of a driven holddown belt has always been encumbered with a number of disadvantages. The belt must be mounted for continuous travel in the manner of a conventional conveyor belt system and, therefore, must also include a separate belt drive. The holddown belt also must necessarily overlie the entire surface of the double face web through the heating section and, as a result, may actually inhibit the escape of moisture from the web as it dries. Also, the edges of the belt which overhang the edges of the corrugated web tend to crush the edges and also undesirably run in contact with the heating surfaces laterally beyond the moving web.
More recently, a double backer has been developed in which the driven holddown belt has been eliminated. A stationary holddown mat is supported by its upstream and downstream ends which are vertically adjustable to allow a selected portion of the mat to hang in catenary fashion on the upper surface of the corrugated double face web traveling through the heating section. The web is typically pulled through the heating section by a downstream vacuum conveyor.
Systems utilizing moving holddown belts are both cumbersome and costly. The improved stationary direct contact holddown systems, though providing significant improvements over holddown belt systems, require a web drive system with fairly high operating power requirements.